Special Report Design and Construction of Linn Cove Viaduct Jean M. Muller Chairman of the Board and Technical Director Figg and Muller Engineers, Inc. Tallahassee. Florida James M. Barker Assistant Vice President/Projects Figg and Muller Engiineers, Inc. Mt. Pleasant, South Carolina he Blue Ridge Parkway, owned by so visitors will have an unobstructed Tthe United States National Park Ser- view as they journey through the 479 vices (NPS), is a unique transportation/ mile (755 km) park. recreation network. Built along the crest Construction of the Parkway began in of the beautiful Blue Ridge Mountains 1933 and was completed in the late in North Carolina, the 469 mile (755 km) 1950's with one exception: a 5-mile (8 long highway attracts millions of vaca- kin) length around privately owned tioners each year with its beautiful Grandfather Mountain, which is one of scenery and facilities for camping, hik- the leading tourist attractions in North ing and picnicking. Carolina, The National Park Service had Through the years the NPS has made settled numerous location and right- every effort to preserve the natural of-way problems through the years, but beauty of the roadway and to provide ac- none approached the magnitude of that cess to adjacent areas with parking areas centered on Grandfather Mountain, and nature trails. All highway construc- The owner of Grandfather Mountain tion is in accordance with the "park con- was insistent that construction of the cept." For instance, fill must be placed Parkway must not harm or be obtrusive and granite clad retaining walls built to the natural scenic beauty found there. before the side of a mountain can be cut. Since the owner shared the same All bridges are built with stone-clad philosophy, the Federal Highway Ad- curbs and wingwalls to blend with the ministration (FHWA), which supplies surrounding mountain landscape. engineering services for the Parkway, Bridges must also have open handrails studied alignments and alternate routes 38 Several innovative design and construction techniques were used to build the Linn Cove Viaduct — a 1243 ft (379 m) long, S-shaped precast prestressed segmental concrete bridge — situated in one of the most scenically beautiful regions of the United States. PCI JOURNALISeptember-October 1985 39 Fig. 1. Finished view of the Linn Cove Viaduct set against the beautifully rugged Blue Ridge Mountains. for approximately 10 years. the site, it generally occurred below Finally, a route which could not be a relatively shallow overburden of seen from the mountain top was chosen soil surfaced with decomposed vegeta- around the eastern mountainside. To tion. minimize construction damage to the The structure proposed for this project area, the roadway alignment was raised was a precast post-tensioned segmental above the mountainside by use of a via- concrete box superstructure, eight spans duct structure. in length with both horizontal and verti- Since building the viaduct structure cal curvature conforming to the natural by conventional methods of bridge con- topography. Progressing from south to struction would have done irreparable north, the spans are 98.5, 163 ft (30, 48 damage to the environmentally sensi- m), four at 180, 163 and 98.5 ft (55, 50 tive area, the search for an acceptable and 30 m) in length. Each span is com- construction method became a chal- prised of 9 ft (2.7 rn) deep single cell lenging engineering problem. boxes of 8.5 ft (2.6 m) nominal length located between 5 ft (1.5 m) long pier DESIGN segments. The Linn Cove Viaduct is probably The site within the limits of the Linn one of the most complex bridges ever Cove Viaduct (Fig. 1) is a rugged and built; certainly, it must be the most steeply sloped terrain with relatively complicated concrete segmental bridge heavy ground vegetation and boulder ever constructed. Three major factors out-croppings. A natural stream crosses contribute to its complexity: environ- the bridge near the northern end. The mental constraints and inaccessibility of scenic location, with its protruding the site and the vertical and horizontal boulders and the requirement for their alignment. protection and preservation, was the The bridge was literally built on the major factor to be considered in the de- side of a mountain which had to remain sign. Although rock existed throughout in its natural state. There was only one 40 Fig. 2. Construction overview of Linn Cove Viaduct. This bridge is the first segmental structure in the United States to incorporate progressive placing erection. way in or out, namely, over the com- mations. pleted portion of the bridge. The hori- The bridge is 1243 ft (379 m) in over- zontal alignment includes spiral curves all length with a curb-to-curb roadway going into circular curves with radii as width of 35 ft (11 m). The final roadway small as 250 ft (76 m) and with curvature surface is a 2 in. (51 mm) bituminous in two directions, which gives the overlay with waterproofing membrane. bridge its S shape (Fig. 2). A small por- The structure was designed in 1979 in tion of the bridge is on a horizontal tan- accordance with the AASHTO Standard gent. The superelevation goes from a Specifications for Highway Bridges full 10 percent in one direction to a full (Twelth Edition 1977) except for those 10 percent in the other direction in 180 design and construction techniques ft (55 m) transitions and part way back particular to post-tensioned segmental again within the length of the bridge. construction. All superstructure and No two of the 153 superstructure seg- substructure precast segments were de- ments have the same dimensions, and signed for a 28-day concrete strength of only one of the segments in the entire 6000 psi (41 MPa) with 4000 psi (28 bridge is straight. When the vertical MPa) specified for release of the forms. curve and tangential alignment are con- In addition to the longitudinal post- sidered, the Linn Cove Viaduct includes tensioning tendons, the top slab was de- almost every kind of alignment signed to be transversely post- geometry used in highway construction. tensioned, All permanent post- The foundations consist of cast-in- tensioning was done with' in. (13 mm) place abutments at each end and seven diameter, 270 ksi (1863 MPa), low re- intermediate precast segmental box laxation strand tendons. piers bearing on 20 ft (6.1 m) diameter To minimize the environmental im- footings. Both the abutments and pier pact on the construction site, the major footings are supported on reinforced 9 portion of the bridge was to be built with- in. (229 mm) diameter microshafts out the use of access roads. The only drilled into the underlying rock for- permitted construction road was from PCI JOURNAL September-October 1985 41 the south abutment of the second pier, a No trees other than ones directly be- distance of approximately 260 ft (79 m). neath the bridge were allowed to be cut. From this pier to the end of the bridge, Each tree had to be evaluated separately the construction was to be from the pre and approved for cutting. All foliage ad- -viously completed portion of the deck. jacent to the bridge had to be protected li TT rTTT'T'T -FT-F IL LLILLLi LJ_LJ Permanent __.4.f[ Permanent Pier Support ^ Pier J,LL Fig. 3. Schematic view of the progressive placing erection method. HALF SECTION AT TYPICAL HALF SECTION POST-TENSIONING BLOCK THRU SEGMENT Fig. 4. Typical cross section of precast segment. The webs and bottom stab were thickened to carry the stresses due to the extreme curvature. Both temporary and permanent post-tensioning tendons were anchored at the intermediate stiffeners shown on the left side of the drawing. 42 by a silt fence located along the entire beautiful site of Linn Cove, it was de- length of the bridge. Any construction cided to use a span-to-depth ratio of 22 debris on the outside of the silt fence to provide a graceful structure. How- had to he immediately retrieved. ever, after analysis of this cross section, None of the boulders could be de- it was decided to increase the depth of faced during construction, except in in- the box and the web thicknesses be- stances of rock bolting. The boulders cause the shallower depth structure were covered to prevent concrete, grout would have required a significant or epoxy stains. Any extraneous material amount of high strength post-tensioning on the boulders was immediately steel, Fig. 4 shows the final cross section cleaned off. adopted. The streams flowing across the bridge Cantilever construction with large alignment were protected from siltation equipment weights applied at the free or other contamination. Water quality end induces high compressive stresses was constantly monitored. in the bottom slab. This in combination Because of the severe environmental with shear stresses resulting from the restrictions, the bridge could not he torsional moment and the longitudinal constructed by conventional methods shear force, results in a bottom slab and a system was devised in the design thicker than the usual 12 in. (305 mm) to construct it from the top. This re- for the typical section and a web width quired a progressive scheme which en- of 18 in. (457 mm). abled segments of the bridge to he Through the use of Figg and Muller's transported across the previously built exclusive Bridge Construction (BC) deck and assembled into final position. Program, it was possible to analyze the Precast concrete was chosen over structure with a discrete model corre- cast-in-place segments because the re- sponding to the joints between seg- gion has a relatively short construction ments (153 segments).